How Do You Illustrate Waveforms for Different 8-QAM Modulation Techniques?

[icednyior]

Homework Statement

Illustrate in wave forms for data “101001000111010011110100” by using the following modulation techniques:
(a) 8-QAM (2 bit ASK + 1 bit PSK)
(b) 8-QAM (2 bit PSK + 1 bit ASK)
(c) 2 bit PSK + 1 bit FSK

2. The attempt at a solution
*attached* apparently I got it all wrong. I am such a noob Would appreciate any help on this. thanks :D

 

[berkeman]

First, can you draw the I-Q diagrams for each of those encoding schemes? I don't think I could draw the time-domain waveforms without first sketching the I-Q diagrams...

 

[icednyior]

First, can you draw the I-Q diagrams for each of those encoding schemes?

we weren't given the I-Q diagrams. I only know how to do the ASK, PSK and FSK diagrams.

 

[berkeman]

The I-Q diagrams at the bottom of this wikipedia.org page may help...

http://en.wikipedia.org/wiki/QAM

 

[icednyior]

According to the email from my lecturer, he said

it is all about representing a group of bits with a wave form. If you are using 256-QAM, it means you have 256 waveforms to represent any combination of 8 bits of data. 2 to the power of 8 is 256.

obviously in you assignment, it is a 3-bit modulation. 2^3=8, so you will have to define 8 different waveforms to represent a group of 3 bits data.
Each waveform will represent 000, 001, 010, 011, 100, 101, 110, 111 respectively.
Each waveform will be different from each other based on the 3 properties of waveform: Amplitude, Phase & Frequency.
If ASK, then the amplitude is different
If PSK, then the phase is different
If FSK, then the frequency is different
If ASK + PSK, then the combination of Amplitude and phase are different.
First, define the 8 sets of waveform and bit-group respectively

Then, If you data is given as 101110111, you can split it up to 101, 110 and 111.
lastly, use the defined waveform to represent each of them...

how do i know what wave form to represent for each group of 3 datas? do i just assign it myself. And what is the difference between 1 bit ASK and 2 bit ASK? Help! I am really desperate to get the answer to this question. *cries*

 

[berkeman]

1 bit ASK is on-off keying. So a 0 might be represented by a flat line, and a 1 represented by one period of a sine wave. 2-bit ASK means that the amplitude can take on 4 values, say from zero to full scale in even steps. Although, as a practical matter, it is best not to use zero amplitude for the bottom of the ASK. You typically will pick a maximum modulation depth less than 100%. So for 1-bit ASK, you might pick amplitudes of 0.5 and 1.0 for the single cycle sine waves. And so on. There are optimal values for these amplitudes, based on channel characteristics. But in your current problem set, I don't think they are asking you to optimize much -- just understand the basic concepts.

So for 8-PSK, you would draw a single cycle sine wave for the 000 value, and phase shift it by 2PI/8 for the 001 value, etc. I'm not sure how they want you to connect the individual timeslots in your waveforms -- you can see that some of the transitions make for some pretty ugly overall time domain waveforms. But there are ways to handle that also (have you learned those yet?).


EDIT -- fixed "2PI/8"

 

[chroot]

Well, in truth, he didn't give you a necessary piece of data: the map between a symbol (like 101) and the corresponding point in the constellation.

He probably wouldn't mind if you assign symbols to points in the constellation yourself, but, honestly, he didn't give you enough information to unambiguously answer the question.

- Warren